How dull it must be, in their minds. How tedious, to not look at someone and immediately understand them. To live out their days, oblivious to the inner workings of humanity. To Sherlock, it is basic. Profiling someone, putting two and two together in a heartbeat - they are the blood in his veins, the air in his lungs. Without them, he feels like he's drowning. Without them, he is restricted - trapped, forced to think along the same lines as the rest of them; those of a lesser intelligence than he. All it does is cement his loathing for the general populace.
Sherlock Holmes doesn't believe in emotions, and certainly not friends - but maybe, just this once, he'll make an exception. Because watching John as he is now, he finds no contempt. The only friend he's ever had stands metres away, in front of an empty grave, begging. Pleading. It's a show of weakness, and Sherlock tries to find disgust. But there's nothing; no contempt, no disgust. Not even pity, which he thinks is the appropriate reaction. Just resignation and apathy. After all; there's a reason he doesn't make friends. And now John Watson is finding out the hard way.
John turns away from the headstone, and he watches silently. He wants to call out, to let John know that he's alive. Because if there's anyone who deserves to know, it's him. Not Lestrade. Not Mrs Hudson. Not Mycroft. John is the only one that matters. It is Dr Watson, after all, who has followed him with blind faith from their first meeting; the only one who has ever cared about Sherlock's reputation. John defends his honour, when even Sherlock himself has tried to tarnish it. The thought makes him wince, and he feels - odd. Mournful.
He stares at John's retreating figure, and the contempt is back. But not for his old roommate, the only friend he's ever had. Rather, the contempt he feels is directed at himself. He's allowed himself to become close to John; not even realising the error until it is far too late. He's weakened himself, and it's disgusting.
There's a reason that Sherlock Holmes doesn't make friends. John disappears in the distance, and he gulps. It's pitiful that he has to remember this the hard way; he can no longer claim aloofness. Now, he is level with the rest of humanity - vermin and idiots alike.
How shameful it is, Sherlock realises, to be in their minds.